In this new age of light, polymers, especially photofunctional polymers, are key materials in the field of optoelectronic applications, such as photolithography techniques, luminescent materials, optical communications, light-emitting diodes displays, and photovoltaic cells, and high-resolution light-driven three-dimensional (3D) printing. These photofunctional applications are based on the dual nature of light: wave and particle. This chapter provides an overview of the fundamental properties of light, which are strongly related to the topics in the sections following: photoresist materials, stimuli-responsive luminescent polymers, plastic optical fibers, polymer solar cells, and polymers for 3D printing.